Astrophysics > Background Material

Some plots and diagrams that can be used with appropriate credit

Feedback from Growing Supermassive Black Holes (AGN)

From Harrison & Ramos Almeida 2024, Galaxies, 12, 17, arXiv:2404.08050

Growing supermassive black holes grow via accretion events. During these events they light up as Active Galactic Nuclei (AGN) and add mass to the black holes - region 1 in the figure. Energy is then released by AGN in multiple forms (see below), which can couple to the gas in and around their host galaxies - region 2 in the figure. This can have a process of self-regulation (i.e., regulate future black hole growth). It can also have an impact on the host galaxies, potentially through a local and transient impact on the host galaxies star formation - region 3 in the figure - or more significantly, the cumulative impact of multiple AGN events can have a long-term impact on the host galaxies properties - region 4 in the figure.

It is crucial to consider AGN as events and not objects, which are permanent in time, when considering their role in the evolution of galaxies.

From Harrison & Ramos Almeida 2024, Galaxies, 12, 17, arXiv:2404.08050

There are multiple mechanism by which AGN can inject energy into the gas in and around their host galaxies.

This schematic diagram summarises the primary modes of energy injection from AGN which are accreting at very low accretion rates, and are inefficient at producing radiation, compared to those which are at high accretion rates and are efficient at producing radiation. In the low accretion regime, jets are often considered to be the dominant mechanism, however for high accretion rate AGN jets, radiation and winds are all possible dominant mechanisms.,

From Harrison & Ramos Almeida 2024, Galaxies, 12, 17, arXiv:2404.08050

AGN driven outflows occur over multiple spatial scales, multiple time scales and multiple phases. This figure summarises these scales and phases in a 1D projection. Outflows can be launched around the accretion disk before propagating through the circumnuclear region, interstellar medium and even region the circumgalactic region and intracluster/intragroup medium. These outflows can be found in temperatures from 10s of Kelvin through to millions of Kelvin in the hottest phases.

From Harrison et al. 2018, NatAs, 2, 198, arXiv:1802.10306

The curves show the percentage of all refereed publications on SAO/NASA ADS that have abstracts containing the combination of keywords shown in the legend each year. Abstracts discussing AGN or quasar outflows were growing relatively steadily over time since ~1990; however, since ~2000 the number of abstracts mentioning AGN feedback has been growing rapidly. We relate this to: (1) three papers that characterise M_(BH)-host galaxy relationships that are in the top-100 of all-time astronomy publications (i.e., top 0.009\% citation counts based on SAO/ADS; Magorrian+98; Ferrarese+00 and Gebhardt+00) and (2) a series of galaxy formation analytical models (e.g., Silk & Rees 98), semi-analytical models (e.g., Benson+03) and hydrodynamical simulations (e.g., DiMatteo+05) where AGN-driven outflows are required to explain these and other observables of galaxy populations. It has been become increasingly popular to attempt to derive observed outflow properties to test AGN feedback models.

From Harrison 2017 - Nature Astronomy Review

The figure shows the ratio of stellar mass to halo mass as a function of halo mass for three different runs of a simulation (Somerville+08) and for the semi-empirical relationship (Moster+13). The shaded region shows the 16th and 84th percentiles of the fiducial model that includes energy injection from AGN and star formation (SF). The right y-axis shows the efficiency for turning baryons into stars (M_stellar/fb*M_halo; where the factor of fb=0.17 is the cosmological baryon fraction). The impact of including star formation feedback in the model is to reduce the efficiency of converting baryons into stars in low mass haloes. For massive haloes, energy injection from AGN is required in order to reduce these efficiencies. Such effects are required in the model in order to reproduce many observable properties of the massive galaxy population.

From Harrison 2017 - Nature Astronomy Review

A schematic diagram to illustrate the relationships between fuel supply, galaxy growth and black hole growth. Both AGN and star formation are fuelled by cold gas that originates from a shared (potentially hot) gas reservoir inside the galaxy halo. This gas reservoir can be fed by gas-rich mergers, by recycled material from internal galactic processes and by accretion of gas from intergalactic material. The amount of gas and the ability for this gas to cool determines the amount of usable fuel that can be used for feeding black hole growth and star formation. In the case of providing the fuel for black hole growth the material has the additional challenge of losing sufficient angular momentum to reach the inner sub-parsec region of the galaxy. Both processes are known to inject energy and momentum (via radiation, winds and jets) that can reduce the availability of usable fuel through ionising, heating, shocking or expelling material, and hence provide self-regulatory feedback mechanisms. A key component of most galaxy formation models is that these two processes can also have a positive or negative impact on the usable fuel supply for the other process (black and grey arrows).

From Harrison thesis (2014), available here

Another schematic diagram to illustrate how AGN activity (BH growth) and star formation (galaxy growth) could be connected. If either one of these processes injects energy or momentum into their surroundings (i.e., provides the source of heating or outflows) future AGN activity and star formation could be enhanced or suppressed (i.e., the impact). The overall results of these processes could affect the observed properties of galaxies, BHs and the gas in the larger scale environment (i.e., the intergalactic medium).

AGN Spectral Energy Distributions (SEDs)

Original from Harrison thesis (2014), available here

Also used in the AGN reviews: Hickox & Alexander, ARA&A (2018) and Alexander et al. 2025

A schematic representation of an AGN spectral energy distribution (SED) loosely based on the observed SEDs of radio-quiet quasars. The black solid curve represents the total SED and the various coloured curves (with an arbitrary offset) represent the individual components. The SED of an AGN in the mm-FIR regime is uncertain; however, it is widely believed to have a minimal contribution (to an overall galaxy SED) compared to star formation, except in the most intrinsically luminous quasars and powerful radio-loud AGN. The primary emission from the AGN accretion disk peaks in the UV region. Radio-loud AGN have radio emission that can be several orders of magnitude higher than radio-quiet AGN (shown with the labelled orange line). Also shown is an example radio-UV SED of a starburst galaxy (grey curve; the SED is of M82 taken from the GRASIL library; Silva et al. 98).

Adapted from my thesis for publication in the AGN review: Padovani et al. (2017)

An adaption of the SED from above, now with two blazar SEDs overlaid: a high synchrotron peaked blazar (HBL, based on the SED of MKN 421) and a low synchrotron peaked blazar (LBL, based on the SED of 3C 454.3).

AGN Identification: Optical emission-line diagnostics

From Harrison thesis (2014), available here

Plots to highlight typical optical emission-line diagnostics used to identify AGN. All data shown are taken from the SDSS DR7 database for extragalactic sources with z<0.4. Left: Emission-line flux ratios of [O III]5007/Hbeta versus [N II]6584/Halpha for sources with FWHM(Halpha)<1000 km/s (to remove the majority of Type 1 AGN). The dashed lines are taken from Ho et al. 1997 and Kauffmann et al. 2003 to identify different source classifications (i.e., Type 2 AGN [shown in red]; LINERS and H II galaxies [shown in black]). Right:[O III]5007/Hbeta versus FWHM(Halpha). Broad Halpha emission-line profiles (i.e., FWHM >~1000 km/s) may indicate the presence of an AGN broad line region. It is possible for reasonably broad emission lines (i.e., 1000 km/s ~< FWHM ~< 2000 km/s) to be produced in the NLR due to outflows therefore care needs to be taken when classifying AGN with these ``intermediate'' line widths.

Integral Field Spectroscopy

From Harrison thesis (2014), available here

Schematic diagram of a data cube. A data cube provides information in three dimensions: two spatial dimensions (i.e., an image in [x,y]) and a third dimension of wavelength (equivalently velocity or frequency). It is therefore possible to obtain an image of the target at a single wavelength or collapsed over wavelength slices. Furthermore, at every spatial pixel of the datacube a spectrum can be extracted.

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